On the binding of electrons to nitromethane: Dipole and valence bound anions

Conventional ͑valence͒ and dipole-bound anions of the nitromethane molecule are studied using negative ion photoelectron spectroscopy, Rydberg charge exchange and field detachment techniques. Reaction rates for charge exchange between Cs(ns,nd) and Xe(n f ) Rydberg atoms with CH 3 NO 2 exhibit a pronounced maximum at an effective quantum number of n*Ϸ13Ϯ1 which is characteristic of the formation of dipole-bound anions ͓͑CH 3 NO 2 ͒ϭ3.46 D͔. However, the breadth ͑⌬nϷ5, FWHM͒ of the n-dependence of the reaction rate is also interpreted to be indicative of direct attachment into a valence anion state via a ''doorway'' dipole anion state. Studies of the electric field detachment of CH 3 NO 2 Ϫ formed through the Xe(n f ) reactions at various n values provide further evidence for the formation of both a dipole-bound anion as well as a contribution from the valence bound anion. Analysis of the field ionization data yields a dipole electron affinity of 12Ϯ3 meV. Photodetachment of CH 3 NO 2 Ϫ and CD 3 NO 2 Ϫ formed via a supersonic expansion nozzle ion source produces a photoelectron spectrum with a long vibrational progression indicative of a conventional ͑valence bound͒ anion with a substantial difference in the equilibrium structure of the anion and its corresponding neutral. Assignment of the origin ͑vЈϭ0, vЉϭ0͒ transitions in the photoelectron spectra of CH 3 NO 2 Ϫ and CD 3 NO 2 Ϫ yields adiabatic electron affinities of 0.26Ϯ0.08 and 0.24Ϯ0.08 eV, respectively